we see how new darkfallen customizations were given to the horde and the alliance. And although I personally don't like that the blood elves have darkfallen as a customization because it ruins the blood elves lore. I do think that if the void elves and night elves have new customizations, the nightborne also deserve new customizations and I think that what best suits the lore is felborne! the felborne exist and warlock is a class of the nightborne.
a warlock nightborne would look great with felborne customization